1. Technical Field
The disclosure discussed herein generally relates to a spectral image acquiring apparatus for acquiring a spectral image.
2. Description of the Related Art
Spectroscopes are widely used for acquiring optical spectra. Typically, a spectroscope separates incoming light into plural wavelength components by using a prism or a diffraction grating, and detects the optical intensity of the wavelength components with a light-receiving element. However, such a general-purpose spectroscope cannot acquire an optical spectrum of the incoming light in association with positional information of the incoming light.
In recent years, much attention has been focused on a spectral image acquiring apparatus capable of acquiring a spectral image that two-dimensionally represents wavelength components of light from various points within a predetermined imaged area in association with the various points. The acquired spectral image represents a two-dimensional distribution of the wavelength components of the various points in the imaged area. For example, the spectral image represents the wavelength components of the various points within the imaged area by gradational differences.
Methods for acquiring spectra may be categorized into three types; namely, one employing a wavelength selective filter (such as a bandpass filter, a low-pass filter, or a high-pass filter); one employing a dispersive element such as a prism or a diffraction grating; and another using Fourier spectroscopy. Various spectral image acquiring apparatuses employing such methods have been proposed.
As an example of the apparatus capable of acquiring a spectral image by using a wavelength selective filter, Japanese Laid-open Patent Publication No. 2005-57541 discusses a spectroscopic camera head. In this spectroscopic camera head, incoming light from a subject is received by a two-dimensional imaging element via a wavelength selective filter configured to provide a spectral image of a wavelength component corresponding to the wavelength selective filter. The spectroscopic camera head can dynamically switch transmission wavelengths by using a liquid crystal wavelength-tunable filter as the wavelength selective filter. Thus, by imaging a subject by switching the transmitted wavelength of the wavelength selective filter, plural images with different wavelength components can be obtained. By composing such images, a spectral image that two-dimensionally represents the wavelength components of various points in the imaged area in association with the various points can be acquired.
As an example of the apparatus capable of acquiring a spectral image by using a dispersive element, “Optical Alliance”, JAPAN INDUSTRIAL PUBLISHING CO., LTD., November 1999, pp. 4-9 discusses a planar spectrometer. The planar spectrometer includes an imaging spectroscope capable of simultaneously measuring positional information and spectral information of points located on a straight line. By performing imaging while scanning the imaging spectroscope in a direction perpendicular to the direction in which the points are arranged, a spectral image can be obtained in which the wavelength components of the points within the imaged area are two-dimensionally represented in association with the points.
As an example of the imaging apparatus using Fourier spectroscopy, Japanese Laid-open Patent Publication No. 2005-31007 discusses a spectrometer apparatus that separates incoming light into two optical paths or polarization components. A phase difference is provided to one of the two optical paths or polarization components so as to cause the two optical paths or polarization components to interfere with each other. A resultant detection signal is Fourier-transformed by a computer in order to obtain an optical spectrum. When a spectral image is to be obtained by using the spectrometer apparatus, the phase difference given to the two perpendicular optical paths or polarization components is changed during detection. In this way, a two-dimensional distribution of an optical spectrum can be obtained in a predetermined wide range of wavelengths.
However, it takes a long processing time before a spectral image can be obtained by any of the above methods. Thus, the methods cannot be easily applied for applications for acquiring a spectral image on a real-time basis at high speed.
Specifically, in the method using the wavelength selective filter, the two-dimensional distribution can be obtained only with respect to one wavelength component by a single imaging operation. In order to obtain a spectral image, plural images may be acquired by using different wavelength components and then composing the images. However, this method takes a long time because of the need for the multiple imaging operations and the process of composing the multiple images, resulting in a long processing time for obtaining the spectral image.
In the method using the dispersive element, in order to obtain a spectral image, imaging is performed while scanning the imaging spectroscope, so that the process time for obtaining the spectral image is extended by the scanning time.
In the method using Fourier spectroscopy, in order to obtain a spectral image, detection is performed while changing the phase difference given to the two optical paths or polarization components, so that the time for obtaining the spectral image is extended by the time for detection.